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1. The physical, climatic and economic conditions in Nairobi township are briefly discussed. The factor which appears to be most closely related with the incidence of plague is rainfall. A growing season producing, mainly by virtue of ample rainfall, large crops of cereals, which for various reasons may be kept in storage for long periods and become easily available to the rat population, is invariably followed by plague.
2. The numbers of Indians and Africans are given together with the incidence of the disease in these two types and the localities where the disease originates and persists.
3. The distribution of rat and flea species is outlined and shows an irregular distribution of the species of Xenopsylla. The rat population during outbreaks shows a higher density than during quiescent periods, but not such as to indicate that numerical strength alone is the main factor causing outbreaks of plague in the Rattus population. Higher flea populations follow high densities in the rat population.
4. Attention is drawn to the area where plague originates and persists and to the mixed flea population of the locality. Pumwani and Pangani with almost pure cheopis colonies do not figure in early outbreaks and in them the disease ceases much earlier, but they suffer heavily during epidemics.
Outbreaks of plague in Nairobi are associated with grain storage areas and negligence and improvidence are important contributory factors.
5. The results of the survey have enabled some advance to be made in understanding the distribution of the two main species of Xenopsylla. All Rattus living in earth burrows have been found infested with Xenopsylla cheopis, and Rattus living in roofs or walls with Xenopsylla brasiliensis. These facts explain why such places as Pumwani with its African population has a cheopis colony and the K.A.R. lines have brasiliensis. In the Indian Bazaar with varied facilities for nesting, there is a mixed population of fleas.
The application of these facts to rural areas explains why brasiliensis is almost the sole ectoparasite of Rattus living in thatched roof habitations, and why field rodents such as Mastomys coucha and Arvicanthis abyssinicus living underground are hosts to Xenopsylla cheopis.
